In recent years, a liquid crystal display which has a light weight and consumes a small amount of electric power has been used as a most competitive display that can be used instead of a cathode ray tube. In particular, since a thin film transistor liquid crystal display (TFT-LCD) that is driven by using a thin film transistor independently drives each of pixels, a response speed of the liquid crystal is very high, thus, a high-quality dynamic image can be realized. Accordingly, currently, the thin film transistor liquid crystal display is applied to a notebook computer, a wall-mounted television and the like, and the application range thereof is expanded.
During the production of a typical color thin film transistor-liquid crystal display, a thin film transistor driving device and an ITO transparent electrode are layered on a glass substrate, and an alignment film is then layered thereon to form a lower substrate of a cell. Spacers are formed by using a silant in order to inject a liquid crystal material between inner surfaces of a pair of upper and lower substrates, polarized films are provided on outer surfaces of the glass substrates, and the liquid crystal material is injected between a pair of substrates and cured to produce a liquid crystal display cell.
In the TFT-LCD, in order to use the liquid crystal as an optical switch, it is required that the liquid crystal is initially aligned on the layer on which the thin film transistor is formed at the innermost part of the display cell in a predetermined direction. In order to achieve this, a liquid crystal alignment film is used.
With respect to a method for producing the alignment film, in the related art, a polymer film such as polyimide is applied on a substrate such as glass and the like, and the surface is subjected to a rubbing process by using a fiber such as nylon or polyester in a predetermined direction. However, the rubbing process may cause fine dust or static electricity when the fibroid material is rubbed in conjunction with the polymer film, which may cause a serious problem during the production of the liquid crystal panel.
In order to avoid the problem of the rubbing process, in recent years, an optical alignment process has been studied to radiate light instead of using the rubbing so that anisotropy is induced to the polymer film to align the liquid crystal. The optical alignment anisotropically occurs by reactions such as light isomerization and photocrosslinking of a photosensitive group that is connected to the polymer due to linearly polarized ultraviolet rays. Accordingly, in the optical alignment process, the liquid crystals may be unidirectionally aligned by providing the anisotropy to the surface of the polymer by using the ultraviolet rays.
The polycinnamate-based polymer such as PVCN (poly(vinyl cinnamate)) and PVMC (poly(vinyl methoxycinnamate)) has been mainly used as a representative material of the optical alignment film. However, the polycinnamate-based polymer has a problem in that the optical alignment property of the polymer is excellent but the thermal stability is poor.
For example, Japanese Unexamined Patent Application Publication No. 11-181127 discloses a method of producing a polymer-type alignment film that has a main chain such as acrylate and methacrylate and a side chain having a photosensitive group such as a cinnamate group, and an alignment film produced by using the method. However, the patent is disadvantageous in that since the mobility of the polymer is poor, even though the polymer is exposed to light for a long time, it is difficult to obtain the desired alignment property. The reason for this is that since the photosensitive group which is present in the polymer is restricted by the main chain of the polymer, the group is difficult to rapidly react with the radiated polarized light. Accordingly, since a long time is required to obtain a network polymer, process efficiency is reduced, and if an alignment treatment is not processed with sufficient time, a plurality of unreacted photofunctional groups reduce the light stability and the thermal stability. Therefore, in the case of the liquid crystal display device produced by using the polymer, since the alignment of the liquid crystal is insufficient, a dichroic ratio is low, contrast is reduced, and the anchoring force is reduced. Thus, there is a disadvantage in that it is difficult to obtain a high quality image.